Threaded connections are widely used for effecting a leak-proof joint with joined pipe sections whether it be a conduit joint, coupling joint or other form of fitting. Exemplifying couplings of the compression type for that purpose is the disclosure of U.S. Pat. No. 3,194,592 in which a nut is threaded onto a sleeve for effecting a gasket seal with a contained pipe end. A frequent application for compression couplings is in connection with buried underground piping for which it has been common practice to apply a chemically protective coating on the coupling components either before or after installation.
Prior coating practices have operated reasonably well where extreme care is exercised to insure against any void in the exposed surfaces including that of the engaged threads. Failure to observe adequate care can, however, expose any voided area to corrosive attack and thereby defeat the purpose for which the coating had been applied. At the same time, post-installation coating, particularly in underground installations with which such couplings are commonly used, can be most difficult if not impossible to effect with any degree of reliability. Yet when the components, including the threads, have been precoated, there is always the risk that a coating if too thick will interfere with the threading action and if too thin will flake or otherwise be affected adversely by abrasion in the course of assembling the joint. With the advent of plastic coatings, various constructions have been proposed for enhancing the desired protection. Exemplifying the latter is the disclosure of U.S. Pat. No. 3,799,584 in which the assembled metal coupling is completely shrouded in a protective coating of a polymeric plastic. Yet another approach has been to utilize a plastic-to-plastic thread formation in the assembled joint as disclosed in U.S. Pat. No. 3,915,478.